Nanocrystals made from the material – caesium lead halide perovskite – are said to have a ground exciton state of ‘bright’, indicating the light is emitted more quickly.
“The discovery of such material and the understanding of the nature of the existence of the ground bright exciton open the way for the discovery of other semiconductor structures with bright ground excitons,” said Dr Alexander Efros, an NRL research physicist. “An optically active bright exciton in this material emits light much faster than in conventional light emitting materials and enables larger power, lower energy use and faster switching for communication and sensors.”
The work studied lead halide perovskites featuring chlorine, bromine and iodine. Nanocrystals made of these compounds and their alloys can be tuned to emit light at wavelengths that span the entire visible range, while retaining fast light emission.
Semiconductors emit light when bound pairs of electrons and holes – or excitons – recombine in a process called radiative decay. "In all known semiconductors and semiconductor nanostructures, the lowest energy state for a bound electron-hole pair is a ‘dark’ state,” Dr Efros explained. “This means the material emits light slowly and weakly.”
Because the lowest energy excitons in perovskite nanocrystals are bright, electrons and holes can recombine and emit light up to 20 times more quickly than conventional materials at room temperature and 1000 times faster at cryogenic temperatures.
“In principle, the 20 times shorter [radiative] lifetime could lead to 20 times more intense LEDs and lasers,” Dr Efros contended.
One potential application is free space communications using visible light. “The maximum bandwidth of the communication system is limited by the rate at which LEDs can turn on and off,” Dr Efros noted. “Shorter radiative lifetimes translate directly into faster switching and therefore a higher data transmission rate.”
